1. Field of the Invention
The present invention relates to a method and a sensing device for measuring a component contained in a measurement gas, and more particularly to methods and various gas sensors for determining the concentration of gas components containing bonded oxygen. In particular, the present invention is concerned with a sensor adapted for measuring the concentration of NOx as a component of a combustion gas, and a method for determining the NOx concentration.
2. Discussion of Related Art
Various measuring methods and devices have been proposed for determining the concentration of a desired gas component in a measurement gas. A known method of measuring NOx in combustion gases, for example, employs a sensor having an oxygen-ion conductive solid electrolyte, such as zirconia, and a Pt-containing electrode and a Rh-containing electrode formed thereon. This method utilizes the ability of Rh to reduce NOx, and determines the NOx concentration by measuring an electromotive force induced between the two electrodes. However, this sensor tends to suffer from noise, since the electromotive force varies to a great extent with a change in the concentration of oxygen contained in the combustion gases, but varies a small extent in response to a change in the NOx concentration. In addition, CO or other reducing gas is needed for the Rh electrode to reduce the NOx. Under a lean combustion condition using an excessively small amount of a fuel, a large amount of NOx is generated, which exceeds the amount of CO generated. Thus, the known sensor is not able to make a measurement with respect to a combustion gas produced under such a lean combustion condition.
There is also known a method of measuring NOx by using a pair of cells consisting of an electrochemical pumping cell and a sensing cell, which pair of cells include Pt electrodes and oxygen-ion conductive solid electrolyte, and another pair of cells consisting of an electrochemical pumping cell and a sensing cell, which another pair of cells include Rh electrodes and oxygen-ion conductive solid electrolyte, as disclosed in JP-A-63-38154 and JP-A-64-39545. The concentration of NOx is calculated based on a difference between pumping currents flowing through the respective pumping cells. In further methods as disclosed in JP-A-1-277751 and JP-A-2-1543, a first and a second pair of cells each consisting of an electrochemical pumping cell and a sensing cell are prepared, and a limiting current is measured by a sensor having the first pair of pumping and sensing cells, under the oxygen partial pressure that does not allow reduction of NOx, while a limiting current is measured by a sensor having the second pair of pumping and sensing cells, under the oxygen partial pressure which allows reduction of NOx, so as to measure the NOx concentration based on a difference between the limiting currents of the two sensors. It is also proposed to measure a difference in the limiting currents, using a sensor having a pair of pumping and sensing cells, by regulating the oxygen partial pressure in the measurement gas between two levels, one of which does not allow reduction of NOx while the other allows the NOx reduction.
Referring to FIG. 25 showing the principle of the known methods as described above, a first and a second sensing element 61, 62, which are formed independently of each other, have respective internal spaces 65, 66 which communicate with an exterior measurement-gas space through corresponding diffusion resistance portions 63, 64, and respective electrochemical pumping cells 67, 68 using solid electrolyte. The first sensing element 61 effects pumping of only oxygen under a predetermined diffusion resistance, and the oxygen concentration is obtained by multiplying the pumping current Ip.sub.1 by a current sensitivity coefficient K.sub.1. The second sensing element 62 having an electrode or catalyst capable of reducing NOx effects pumping of both oxygen and NOx under a predetermined diffusion resistance, and a sum of the oxygen and NOx concentrations is obtained by multiplying the pumping current Ip.sub.2 by a current sensitivity coefficient K.sub.2. Thus, the NOx concentration "Cn" is calculated according to the following equation: EQU Cn=K.sub.2 .multidot.Ip.sub.2 -K.sub.1 .multidot.Ip.sub.1
In the above method of measuring NOx, however, a considerably small current flows due to NOx whose concentration is measured, and a considerably large portion of the limiting current is caused by a large amount of oxygen contained in the measurement gas. Therefore, the small current value corresponding to NOx is obtained from a difference between the two large pumping currents Ip.sub.1, Ip.sub.2. Where the method uses only one sensor in which the oxygen partial pressure is regulated as described above, the NOx cannot be continuously measured, and the operating response and measuring accuracy are deteriorated. Where the method uses two sensors having different oxygen partial pressures, a measurement error is likely to occur with a great change in the oxygen concentration of the measurement gas, and therefore this method cannot be employed in automobile applications, for example, where the oxygen concentration in exhaust gases varies to a large extent. This is because the dependency of the pumping current of one of the sensors on the oxygen concentration differs from that of the other sensor. In the case of an automobile running under the air/fuel ratio of 20, for example, the oxygen concentration is generally several percentages of exhaust gases, whereas the NOx concentration is several hundreds of ppm, which is about 1/100 of the oxygen. If the dependency of the pumping current on the oxygen concentration slightly differs between the two sensors, therefore, a difference in the limiting currents due to the varying oxygen concentration becomes larger than a change in the limiting currents due to NOx whose concentration is measured. The known method involves other problems. That is, if diffusion means or small leak formed in the pumping cell is clogged with oil ash in the exhaust gases, the pumping current may be undesirably changed, resulting in reduced measuring accuracy. With a great variation in the temperature of the exhaust gases, the measurement results may involve some abnormality. Further, a difference in the chronological changes in the characteristics between the two sensors may lead to measuring errors, making the sensors undurable for use for a long period of time.
While the oxygen present in the measurement gas causes various problems upon measurement of NOx, as described above, the oxygen also causes similar problems, such as reduced measuring accuracy, upon measurement of gas components other than NOx, and there has been a strong need to solve these problems.